1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device and a switching element used therein, and, more particularly, to an active matrix liquid crystal display device having an excellent display uniformity in addition to low level flickering and a switching element used in the display device.
2. Description of the Prior Art
Thin film transistors (TFTs) are widely used as switching elements in drive circuits for various devices. The use of TFTs in active matrix liquid crystal display devices is particularly remarkable. In case where the semiconductor region of a TFT is formed of amorphous silicon, which is often used in a liquid crystal display or the like, incident light from a backlight or a light source for display generates photocarriers in the semiconductor region. As the photocarriers travel in a channel portion, a light-induced OFF leak current is produced. The light-induced OFF leak current lowers the pixel potential, thus resulting in various undesirable shortcomings, such as reduced luminance of a module, uneven display and flickering.
Some schemes of suppressing the light-induced OFF leak current are disclosed in Japanese Patent Laid-Open No. 26768/1999 and Japanese Patent Laid-Open No. 122754/1995. FIGS. 1 and 2 illustrate a TFT of an active matrix liquid crystal display device according to the art described in Japanese Patent Laid-Open No. 26768/1999. FIG. 1 is a plan view of the TFT, and FIG. 2 is a cross-sectional view of a portion near the TFT in FIG. 1 when the TFT is cut in the channel length direction.
It should be noted that although the plan view shown in FIG. 1 illustrates a pixel of an active matrix substrate operating in a vertical electric field mode, the inventor of the present invention intends to primarily explain the layout pattern of TFT, and therefore, the layout pattern of components other than TFT is only an exemplified one, in other words, not limited to that of the vertical field type of active matrix substrate. For that reason, instead of a vertical field type of active matrix substrate, in order to explain a path of light 119 incident on TFT hereinafter, FIG. 2 illustrates a cross sectional view of an active matrix substrate operating in a lateral electric field mode, which is generally more sensitive to an incident light than the vertical field type of active matrix substrate.
In FIG. 2, a gate line 101, a common electrode 104, a first insulating film 109, an amorphous silicon film 107, an n+amorphous silicon film 108, a source (pixel) electrode 105 and a drain electrode 106 are formed on a first glass substrate 100. Further, a second insulating film 110 and a first alignment film 111 are formed over the gate line, films and electrodes, thereby completing a thin-film-transistor substrate (hereinafter referred to as xe2x80x9cTFT substratexe2x80x9d) 130.
An opposing substrate 140 is formed so as to face the TFT substrate 130 interposing a liquid crystal 116 therebetween. The opposing substrate 140 has a second glass substrate 112 and a light shielding film 113, a color layer 114, a third insulating film 115 and a second alignment film 117 formed in the name order on that side of the second glass substrate 112 which faces the first glass substrate 100.
Furthermore, a first polarizer 121 is adhered to the bottom side of the first glass substrate 100 and a second polarizer 122 to the bottom side of the second glass substrate 112, thereby completing a liquid crystal display panel 150.
As parts of the semiconductor region 107 over the gate line 101, which is sandwiched between the source electrode 105 and the drain electrode 106, are cut away as shown in FIG. 1, the influence of the light that enters the back channel along a path 1 in FIG. 2 is reduced.
FIG. 3 is a cross-sectional view of a TFT of an active matrix liquid crystal display device according to the art described in Japanese Patent Laid-Open No. 122754/1995. Same reference numerals are given to those materials and components, which are the same as those shown in FIG. 2. As a gate electrode is formed so that the width xcex1 of the gate electrode in the channel length direction becomes equal to or greater than four times the distance d from the gate line 101 to the light shielding film on the opposing substrate side as shown in FIG. 3, the light that enters the back channel along a path 1 in FIG. 3 receiving one time reflection is suppressed.
In case of TFTs which are used in a liquid crystal display panel for a monitor, an on-board display or the like, the backlight luminance is increased to meet the demand for high luminance display, the light-induced OFF leak current generated in the semiconductor region by incident light cannot be solved by the existing techniques alone. In case of a panel, which is driven in lateral electric field mode, the issue is more critical because the aperture ratio is low and the backlight luminance should be set higher than that of a TN type, which is driven by a longitudinal electric field.
The above-described conventional methods for reducing light-induced OFF leak current are insufficient to suppress the light-induced OFF leak current and further suppression of light-induced OFF leak current needs to be carried out.
The insufficient effect obtained by employing the above-described conventional methods will be discussed below in more detail. In the case where a double-layer Cr or multi-layer Cr having high reflectance is used as a light shielding film on the opposing substrate, the amount of light reflected by a part of the light shielding film on the opposing substrate is increased relative to the backlight incident toward the opposing substrate from the TFT substrate. Therefore, the art described in Japanese Patent Laid-Open No. 26768/1999 can demonstrate a prominent effect in reducing light-induced OFF leak current by reducing the area of a semiconductor region that receives the reflected light, and the art described in Japanese Patent Laid-Open No. 122754/1995 can demonstrate a prominent effect by widening the gate electrode width to reduce light incident on the back channel accompanied by the reduction of the light itself that is reflected by the light shielding film. However, in the case where a low-reflectance resin which is often used in an active matrix liquid crystal display device driven in a lateral electric field mode or the like is used as a light shielding film on the opposing substrate, since the very amount of the light reflected by the light shielding film reduces because of the low reflectance of the light shielding film, the effect in reducing light-induced OFF leak current becomes relatively smaller, in other words, does not influence seriously even when the above-described two schemes are employed in the device.
Another reason why the above-described two schemes cannot decrease the light-induced OFF leak current so much is that the backlight is reflected between the drain of TFT and the gate electrode multiple times while penetrating through the semiconductor region as indicated by a path 2 in FIG. 2, thereby allowing some light to enter the front channel. Therefore, a new technique is required to suppress the light-induced OFF leak current caused by a light incident along the path 2.
Accordingly, it is an object of the invention to provide an active matrix liquid crystal display device that has less flickering and an excellent uniformity of display as a result of reduction of leak current, which generates problems in the conventional active matrix liquid crystal display device, caused by the light incident on transistors. It is another object of the invention to provide a switching element that has less flickering and an excellent uniformity of display and then, to provide a display device employing such switching elements.
An active matrix liquid crystal display device according to the invention has the following fundamental structure.
The active matrix liquid crystal display device comprises a thin-film-transistor array substrate, an opposing substrate disposed to face the thin-film-transistor array substrate and a liquid crystal sandwiched between the thin-film-transistor array substrate and the opposing substrate.
The thin-film-transistor array of the above-described display device has thin film transistors formed on one surface of a first substrate and a polarizer formed on the other surface of the first substrate, each of the thin film transistors including a semiconductor region formed on an insulating film covering a gate electrode formed on the first substrate, and a source electrode and a drain electrode formed apart from each other on both end portions of the semiconductor region so as to partly overlap the semiconductor region and extending on the insulating film, the source electrode and the drain electrode being formed in such a manner that widths of the source electrode and the drain electrode in a channel width direction are included in a width of the semiconductor region in the channel width direction, while both a planar source-side overlap area constructed by the gate electrode, the source electrode and the semiconductor region and a planar drain-side overlap area constructed by the gate electrode, the drain electrode and the semiconductor region are provided such that an optimal overlap length of one of the source-side and the drain-side overlap areas in the channel length direction is determined for a light incident on a channel portion of each of the thin film transistors to have a light intensity of below or equal to 0.2% of a light intensity of a backlight incident on the first substrate.
A first preferable application form of the active matrix liquid crystal display device of the invention is constructed so that the width of the semiconductor region of each of the thin film transistors in the channel length direction is made wider than that of the gate electrode in the channel length direction and the semiconductor region is formed to extend wider than the gate electrode in the channel length direction.
A second preferable application form of the active matrix liquid crystal display device of the invention is constructed so that the width of the semiconductor region of each of the thin film transistors in the channel length direction is made narrower than that of the gate electrode in the channel length direction and the semiconductor region is formed to be included within the gate electrode in the channel length direction.
A switching element according to the invention has the following fundamental structure.
The switching element comprises a gate electrode formed on a surface of a substrate, an insulating film formed on the substrate and the gate electrode, and a semiconductor region formed on the insulating film and located over the gate electrode, and a source electrode and a drain electrode formed apart from each other on both end portions of the semiconductor region so as to partly overlap the semiconductor region and extending on the insulating film.
In the above-described construction of the switching element, the source electrode and the drain electrode are formed in such a manner that widths of the source electrode and the drain electrode in a channel width direction are included in a width of the semiconductor region in the channel width direction, while both a planar source-side overlap area constructed by the gate electrode, the source electrode and the semiconductor region and a planar drain-side overlap area constructed by the gate electrode, the drain electrode and the semiconductor region are provided such that an optimal overlap length of one of the source-side and the drain-side overlap areas in the channel length direction is determined for a light incident on a channel portion of each of the thin film transistors to have a light intensity of below or equal to 0.2% of a light intensity of a backlight incident on the substrate.
A first preferable application form of the switching element of the invention is constructed so that the width of the semiconductor region of the switching element is made wider than that of the gate electrode in the channel length direction and the semiconductor region is formed to extend wider than the gate electrode in the channel length direction.
A second preferable application form of the switching element of the invention is constructed so that the width of the semiconductor region of the switching element in the channel length direction is made narrower than that of the gate electrode in the channel length direction and the semiconductor region is formed to be included within the gate electrode in the channel length direction.